Based on feedback I'd propose not adding 2119 keyword markup at this time.
The RFC-editor could say that the IETF can request additional markup capabilities (new entities, elements, markup, tags), if IETF changes to processes so they can be useful.
Any proposed additional markup should explain the semantics, how it affects the text, html, pdf, etc. renditions of the document, and the process by which correctness of the markup is validated.
So far the proposals for additional markup require changes to IETF processes, policies, conventions, before they would be really useful. And we shouldn't clutter the XML with features on speculation, as they are likely to be incomplete.
(For example, if you typeset MUST, MAY, and SHOULD in small-caps in the HTML and PDF and as uppercase in text, you might want to have a standard 2119 boilerplate in the intro that notes the typesetting convention as referring to 2119. )
Larry